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Open Task Manager and see what uses a lot of memory and CPU resources. If it's something you don't recognize, then try to remove it. Check your page file usage too. If it's close to 2GB, then you might need to get an extra gigabyte of memory.

Practically any firewall other than the Windows firewall makes the computer quite laggy. Avira has seemed to be the least laggiest to me. Norton was awful, though. The biggest threat to a computer is the one between the screen and the chair. If you can keep yourself from downloading random programs from random sites, then you'll most likely be fine without any firewall/AV.

Was there anything you installed (hardware or software) before your computer became slow? That might be the clue. If you don't have a clue, try increasing the page file as already suggested and if you're using Vista you could use a flash drive for ReadyBoost. And maybe upgrade your RAM. Also, a great anti-virus program I can recommend that uses only few memory is Prevx.